The basis for the Long Term Evolution (LTE) Hybrid Automatic Repeat Request (HARQ) mechanism is multiple stop-and-wait protocols, each operating on a single transport block. In LTE, multiple stop-and-wait processes are applied in parallel to allow continuous transmission of data. There is one HARQ entity per terminal and per component carrier. Spatial multiplexing is supported with one HARQ process per transport block transmitted in parallel. The HARQ processes belong to the same HARQ entity, but have independent HARQ acknowledgements (ACKs).
An important part of LTE HARQ is the use of soft combining, which implies that the receiver combines the received signal from multiple transmission attempts. When using soft combining, the erroneously received packet is stored in a buffer memory and is later combined with one or several retransmissions. The decoder is operating on the combined soft buffer, thereby producing a more reliable output than if only a single transmission would have been used.
HARQ with soft combining is typically based on Incremental Redundancy (IR), which includes Chase Combining (CC) as a special case. With IR, each retransmission may be different to the previous transmission, but all retransmissions represent the same information bits. Whenever a retransmission is needed, the retransmission typically uses a different set of coded bits than the previous transmission. The receiver combines the soft information of the first transmission with the soft information of the retransmission. If exactly the same coded bits are used for the first transmission and the retransmission, i.e. CC, the combined soft information corresponds to a codeword with the same length as the first transmission. If any bits that were not part of the first transmission are included in the retransmission, the combined soft information corresponds to a longer codeword with lower code rate. In LTE, the (re)transmissions and IR are based on a circular buffer together with different Redundancy Versions (RVs). The LTE turbo code is a systematic code of rate 1/3, i.e. ⅓ of the coded bits are systematic bits, or information bits, and ⅔ of the coded bits are parity bits. A codeword, after sub-block interleaving, [s0, s1, . . . , sk-1, p01, p02, . . . , pk-11, pk-12], is put into a circular buffer. Each RV indicates a starting point in the circular buffer. To generate n bits of redundancy version T, n bits are read clock-wise from the circular buffer, starting at the position indicated by RV T and wrapping around to s0 if the end of the codeword is reached. The number of bits n to be transmitted can be determined from the scheduling information. FIG. 1 is a schematic description of the LTE circular buffer for rate matching.
When the receiver combines soft information, it is important to know when new information arrives and the soft buffer should be cleared instead of being combined with the latest transmission prior to decoding. The soft information for a coded bit can, for example, represent the Log-Likelihood Ratio (LLR) of the coded bit, or a scaled version of the LLR. Soft information is usually quantized in the soft buffer memory, so that the soft value of each coded bit is represented by a finite number of bits. Clearing the soft buffer for a coded bit could, for example, entail setting the representation in the soft buffer memory to a combination that represents an LLR of 0. In LTE, an explicit New Data Indicator (NDI) is included for each of the scheduled transport blocks along with other downlink scheduling information on the Physical Downlink Control Channel (PDCCH).
For downlink data transmission, the NDI is toggled for each new transport block. When the User Equipment device (UE) receives a downlink scheduling assignment, it checks the NDI to determine whether the current transmission should be soft combined with the received data currently in the soft buffer for the HARQ process in question, or if the soft buffer should be cleared. In addition to the NDI, the HARQ process number and the RV are also explicitly signaled in the scheduling assignment for each downlink transmission. Each transport block is acknowledged by transmitting an ACK/Negative Acknowledgement (NACK) of one or two bits on the uplink. An LTE transport block can consist of one or more codeblocks. A Cyclic Redundancy Check (CRC) is appended to the information bits in each codeblock so the receiver can determine with high probability whether a single codeblock is correctly decoded.
In general, RV 0 contains a larger fraction of systematic bits than other RVs, which makes it easier to decode on its own than other RVs. Therefore, RV 0 should be used when transmitting a new transport block.
The above description focuses on the existing LTE mechanisms, but the relevant parts may also be true for the new New Radio (NR) standard as well. In particular:                The Low-Density Parity-Check (LDPC) codes proposed for NR are easier to decode when the receiver has access to a large fraction of the systematic bits than to mainly parity bits, and they support IR HARQ in a similar way as the LTE turbo codes.        Rate matching based on a circular buffer is proposed for NR.        